1. Field of the Invention
The present invention relates to a color separation method and an image processing apparatus, and in particular to color separation in converting an image signal into a signal of an amount in a color material such as ink or toner.
2. Description of the Related Art
There is conventionally a case where cyan (C), magenta (M), yellow (Y) and black (K) as fundamental color materials of a printing apparatus and in addition to them, light color cyan (Lc), light color magenta (Lm) and the like as light color materials are used to perform recording. In this case, for effecting the color separation to provide color material signals containing a light color, there is known a method of effecting color separation of a signal of a dark color material having a color into a color material signal of each of dark and light colors. In addition, as an image processing apparatus of effecting such color separation, there is known a method as described in Japanese Patent Laid-Open No. 2003-110864. According to this method, by producing an ink color separation table for setting a UCR amount or a BG amount most suitable for each hue, it is possible to effect the color separation using a color reproducibility region at a maximum in an image forming device such as a color printer. In addition, this method can absorbs linear characteristics occurring when a plurality of ink is blended in color, therefore making it possible to reduce influence of granularity due to black without distorted characteristics in lightness, hue and color saturation.
FIG. 23 is a diagram for determining a color material amount of each of a dark color material and a light color material in the conventional dark and light color separation processing. The same figure shows an ink amount for printing a patch of each dark and light color cyan corresponding to a grid of 9×9=81. The lateral direction thereof shows that an ink amount of dark color cyan changes in a range of from Cg 00 to Cg 08 and the longitudinal direction thereof shows that an ink amount of light color cyan changes in a range of from Lcg 00 to Lcg 08. In addition, a slant broken line shows an equal ink amount line in the total ink amount of the dark and light color ink. Further, a slant solid line shows an equal density line in the density of a patch, which is measured in color by printing the patch and then is plotted. Based upon these lines, the dark and light color separation processing is performed from intersection points between the equal ink amount line and the equal density line.
In general, for enhancing accuracy of dark and light color separation, it is required to increase the grid number on a cross patch. This case, however, leads to an increasing search amount in the intersection points between the equal ink amount line and the equal density line in proportion to the grid number. As a result, time required for the dark and light color separation processing becomes longer.